Speaking at Holyrood Mr Neil said: "I can report that NHS Greater Glasgow and Clyde have now been able to resolve the problem with the server and incrementally reloading users back on to the system.

"It would appear that no data appears to have been lost.

"Around 50 per cent of users now have access to the system, and the remainder should have access by later this evening.

"However, the system is being closely monitored to ensure it remains robust."

I hope that no patients will be put in danger because of this

Dr Jean Turner, Scotland Patients Association

Mr Neil added NHS Greater Glasgow and Clyde were contacting all patients affected and that they would receive treatment as soon as possible.

He said there would be a robust review of IT systems and backup systems across the health service.

Staff from the two software system companies, Microsoft and Charteris, are working with both the health board and Scottish Government staff to “try to get to the root cause of the problem”, he added.

Robert Calderwood, chief executive of NHS Greater Glasgow and Clyde, apologised to patients who had missed treatment or appointments because of the “unprecedented IT issue”.

He added: “I can assure everyone that our senior management team, IT specialists and clinical leads have been working around the clock and will continue to do so until this issue is resolved.

“Thanks to these efforts the vast majority of our services have been maintained and around 7,400 procedures and appointments planned over the past 36 hours have gone ahead as scheduled.

“Unfortunately however over the past 36 hours 564 patients have had their treatment or appointment postponed.”

He added: “Arrangements have also been made to maintain the vast majority of chemotherapy sessions today although a small number of sessions have regrettably been postponed.”

Alex Neil says there will be a "robust review" into NHS computer systems

Labour health spokesman Neil Findlay said the situation was “very worrying” because similar IT systems were used across Scotland.

He added: “Now we must look at other health boards and ensure that patients in other parts of Scotland do not face the same delays we have seen in Glasgow and the wider area.

“We need an independent review of all of IT systems across our health boards so that patients are spared these delays and make clear what contingency plans are in place so we can have confidence in our patient booking system in future.”

Jean Turner, executive director of the Scotland Patients Association, described the IT problems as a “disaster”.

She added: “I do hope that no patients will be put in danger because of this, or put at the end of a queue because of this.”